Rivets

ABSTRACT

A rivet for fastening workpiece members, the rivet including a body comprising a shank extending in a first, longitudinal direction and a head extending in a second, substantially transversal direction, a bore being defined through the shank and the head of the body; and, a stem comprising a main longitudinal portion arranged for insertion into said bore, and an enlarged portion arranged for radially expanding the shank when the enlarged portion is forced into said bore to fasten the workpiece members; the rivet being configured for insertion into respective apertures formed in the workpiece members; and the rivet being further configured such that when the workpiece members are fastened, the head of the body of the rivet is displaced relative to the shank to engage with the stem to lock the stem in place relative to the body and/or the workpiece members.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of InternationalApplication PCT/EP2016/068943 with an international filing date of Aug.9, 2016 which claims priority from United Kingdom Patent Application No.GB1514543.6, filed Aug. 14, 2015, the disclosure of which isincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The invention relates to rivets (more accurately, rivet assemblies), forfastening workpiece members. In particular, it relates to blind rivets.More particularly, it relates to blind rivets of the break-stem type.Rivets according to the present invention are capable of providinglocked fastening, i.e. a type of fastening whereby parts of the rivetsare locked together to reduce or eliminate relative movement betweenthose parts independently or in addition to interactions due to thefastening itself.

BACKGROUND OF THE INVENTION

Blind rivet assemblies (henceforth, “rivets”) of the type described inthis application are used to fasten together workpiece members.Workpiece members may, for example, take the form of sheets of materialor interconnecting members, such as tubular members, which need to befastened together so that their relative movement is constrained orprevented.

To fasten the workpiece members together, a blind rivet is passedthrough corresponding apertures in the workpiece members and then set inposition using a tool which applies a pulling force to a stemcomponent/part of the rivet (henceforth, “stem”). The pulling forceapplied to the stem causes deformation of a body component/part of therivet (henceforth, “body”) on the “blind” side of the fastener, i.e. onthe opposite side of the workpiece from the side on which the tool isapplied. The deformation of the body typically results in a deformedportion of the body being in contact with the blind side of theworkpiece, while a head of the body is in contact with the other side ofthe workpiece. The workpiece members are thus fastened (i.e. held orclamped together) by and between the deformed portion of the body andthe head of the body.

The strength of such a fastening and the ability of a set rivet tocontinue constraining relative movement of the workpiece members can becompromised if, once the rivet has been set, the body and the stem areable to move relative to each other. For example, if the rivet is abreakstem rivet (wherein a part of the stem is broken off in the courseof the setting operation so that the stem protrudes less or does notprotrude at all from the body head), the portion of the stem left in thebody may recoil and move within the body, towards the blind side of theworkpiece. The stem can also be caused to move within the body byvibration of the workpiece members or continued stress applied to thejoint. Even a small movement of the stem relative to the body can reducethe effectiveness of the fastening. In some cases, the stem may make itsway completely out of the body. This can in addition significantlyreduce the shear strength of the fastening, i.e. the ability of thefastener to constrain or prevent movement of the workpiece members indirections substantially orthogonal to the longitudinal axis of thefastener.

Locking features that mitigate the aforementioned problems in rivets areknown and come in several different configurations.

Some rivets comprise a shoulder in the body bore which impacts a matingshoulder on the stem to force material, locally, to deform radiallyinwardly into a stem locking groove immediately below the stem shoulderto form the lock. The strength of this lock is only sufficient to resistrecoil for smaller rivets.

Other types of internal locks involve a locking “lip” formed in the headbore. The locking lip is displaced into a locking groove formed in thestem when the rivet is set. However, these locks are difficult to formand also have limited strength.

Other locks may comprise a folding skirt on the stem which his splayedout by a special lipped nose tip of the placing tool, and which locksinto a counterbore in the top of the rivet body head. However, thesedesigns cannot be applied to stems of high hardness, as the foldingskirt would become brittle and liable to wear the corresponding lip ofthe tool nose tip.

In at least some embodiments, the invention aims to go some way towardssolving or at least mitigating the problems set out above.

In at least some embodiments, the invention aims to provide a simplerlocking blind rivet design compared to the prior art.

In at least some embodiments, the invention aims to provide a robustlock suited to relatively large workpiece members which may requirerelatively large apertures and rivets comprising stems having hightensile strength (and thus being particularly prone to recoiling).

SUMMARY OF THE INVENTION

According to an embodiment of the invention there is provided a rivetfor fastening workpiece members, the rivet comprising: a body comprisinga shank extending in a first, longitudinal direction and a headextending in a second, substantially transversal direction, a bore beingdefined through the shank and the head; and a stem comprising a mainlongitudinal portion arranged for insertion into said bore, and anenlarged portion arranged for radially expanding the shank when theenlarged portion is forced into said bore to fasten the workpiecemembers; the rivet being configured for insertion into respectiveapertures formed in the workpiece members; and the rivet being furtherconfigured such that when the workpiece members are fastened, the headof the body of the rivet undergoes displacement relative to the shank toengage with the stem to lock the stem in place relative to the bodyand/or the workpiece members.

In preferred embodiments, the main longitudinal portion of the stemcomprises at least one stem locking feature to facilitate the engagingof the head of the body of the rivet with the stem. The at least onestem locking feature may for example comprises a stem shoulder and/or astem groove.

In preferred embodiments, the head of the body of the rivet comprises atleast one body locking feature to facilitate the engaging of the head ofthe body of the rivet with the stem. The at least one body lockingfeature may comprise an inner body shoulder and/or an inner body groove,for instance.

In some embodiments, the head has an outer curved or domed profile, andsaid head optionally has a flat or curved underhead contact surface forabutment against a corresponding respectively flat or curved surface ofone of the workpiece members.

Preferably, the displacement of the head comprises pivoting of at leasta portion of the head around a region of the head constrained between asurface of one of the workpiece members and an outer rim of a tool forsetting the rivet. At least a substantial portion of the head mayundergo yielding to enable said displacement to lock the stem in placerelative to the body and/or the workpiece members.

Optionally, the body is formed as a single piece and/or is made ofsteel, optionally in an annealed condition. Analogously, the stem may beformed as a single piece and/or be made from steel, optionally in ahardened condition.

The body shank may have a plain cylindrical outer profile, or besplined. Optionally the body shank may include a voided section.

Preferably, the enlarged portion of the stem comprises a tapered sectionarranged to facilitate the radial expansion of at least part of the bodyshank.

Optionally, the workpiece comprises a curved surface for abutment withthe head of the body of the rivet.

According to another embodiment of the invention there is provided amethod of fastening workpiece members using a rivet comprising a bodyhaving a shank extending in a first, longitudinal direction and a headextending in a second, substantially transversal direction, a bore beingdefined through the shank and the head of the body, and a stem having amain longitudinal portion arranged for insertion into said bore, and anenlarged portion arranged for radially expanding the shank, the methodcomprising: inserting the rivet into respective apertures formed in theworkpiece members; forcing the enlarged portion into said bore to fastenthe workpiece members; and displacing the head of the body of the rivetrelative to the shank to engage with the stem to lock the stem in placerelative to the body and/or the workpiece members.

Preferably, the step of displacing the head of the body relative to theshank to engage with the stem to lock the stem in place relative to thebody and/or the workpiece member is performed immediately subsequently,and optionally as a result of a continuation of a same actionresponsible for the step of forcing the enlarged portion into the boreto fasten the workpiece members.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments of the invention will now be described by way ofnon-limiting examples, in which:

FIG. 1 illustrates in cross section a rivet assembly according to anembodiment of the invention;

FIG. 2 illustrates in cross section the fastener of FIG. 1 after it hasbeen installed in a tool and inserted through apertures in a workpieceand is at the beginning of a setting operation. In this figure and FIGS.3 to 7, the pulling jaws and some other components of the tool have beenomitted for clarity;

FIG. 3 illustrates in cross section the fastener of FIGS. 1 and 2 whichis part way through the setting operation;

FIG. 4 illustrates in cross section the fastener of FIGS. 1 to 3 at alater stage in the setting operation;

FIG. 5 illustrates in cross section the fastener of FIGS. 1 to 4 furtherthrough the setting operation;

FIG. 6 illustrates in cross section the fastener of FIGS. 1 to 5 stillfurther through the setting operation;

FIG. 7 illustrates in cross section the fastener of FIGS. 1 to 6 whenthe setting operation has been completed;

FIG. 8 illustrates in cross section a rivet assembly according to anembodiment of the invention after it has been installed in a tool andinserted through apertures in a differently shaped workpiece and is atthe beginning of a setting operation. In this Figure and FIGS. 9 to 13,the pulling jaws of the tool have been included;

FIG. 9 illustrates in cross section the fastener of FIG. 8 which is partway through the setting operation;

FIG. 10 illustrates in cross section the fastener of FIGS. 8 and 9 at alater stage in the setting operation;

FIG. 11 illustrates in cross section the fastener of FIGS. 8 to 10further through the setting operation;

FIG. 12 illustrates in cross section the fastener of FIGS. 8 to 11 stillfurther through the setting operation; and

FIG. 13 illustrates in cross section the fastener of FIGS. 8 to 12 whenthe setting operation has been completed.

DETAILED DESCRIPTION

The present embodiments represent the best ways currently known to theapplicant of putting the invention into practice, but they are not theonly ways in which this can be achieved. They are illustrated, and theywill now be described, by way of example only.

In FIG. 1 a blind breakstem rivet assembly 11 is illustrated. Thefastener 11 includes a body 13 and a stem 15. The body 13 defines anaxial bore along its length, inside which the stem 15 is received andpositioned. The body 13 includes a body shank 17 and a radially enlargedbody head 21.

The body shank 17 extends substantially in a first direction,approximately parallel to the axis of the fastener 11, i.e.approximately parallel to the longitudinal axis of the body 13 and tothe longitudinal axis of the stem 15. Prior to setting of the fastener11 in a workpiece (i.e. when the fastener 11 is in the conditionillustrated in FIG. 1), the body head 21 is arranged such that anunderhead contact surface 22 of the body head 21 (for contacting anouter surface of a workpiece such as the workpiece 19 illustrated inFIGS. 2 to 7 or the workpiece 119 illustrated in FIGS. 8 to 13) liessubstantially in one plane, i.e. is approximately flat. The plane isapproximately orthogonal to the first direction, i.e. approximatelyorthogonal to the longitudinal axis of the body 13 and to thelongitudinal axis of the stem 15. In the illustrated example, the bodyhead 21 has a domed outer surface against which an installation toolwill be pushed during setting of the fastener 11 (see FIGS. 2 to 7 andFIGS. 8 to 13). In other examples, the body head 21 may have a differentouter surface profile, such as a flat or stepped profile.

In the described embodiment, the rivet body 13 also includes a bodylocking feature 23. As the skilled person will appreciate, the lockingmechanism of the rivet is in addition to the main fastening mechanism.In the illustrated example, the body locking feature 23 is a shoulder 23in the interior of the body 13, i.e. on the internal wall defining theaxial bore of the body 13. The shoulder 23 is at the transition betweena region 29 of reduced bore diameter at the head end of the body 13(i.e. the end of the body 13 where the head 21 is located) and a region31 of greater bore diameter closer to the tail end of the body 13 (i.e.the opposite end of the body 13 from the head end).

In other examples, the locking feature 23 may take different forms. Thelocking feature 23 may for example be a protrusion which protrudesradially inwardly from the internal wall of the body 13 (i.e. the walldefining the axial bore of the body 13) and extends for only a shortdistance along the axis of the body 13. The protrusion may extend allthe way round the circumference of the bore of body 13, forming acontinuous, annular protrusion in a plane substantially orthogonal tothe longitudinal axis of the fastener 11, or extend only part way roundthe bore of the body 13. If the protrusion extends only part way roundthe bore of the body 13, there may be several protrusions spaced aroundthe circumference of the bore of body 13.

The body locking feature 23 is configured (e.g. sized and shaped) suchthat it corresponds approximately to a stem locking feature 25. In theillustrated example, the stem locking feature 25 comprises a shoulder 26and a groove 28 in the outer surface of the stem 15. The shoulder 26extends round the circumference of the stem 15, forming a continuous,annular shoulder in a plane substantially orthogonal to the longitudinalaxis of the stem 15. The groove 28 similarly extends round thecircumference of the stem 15, forming a continuous annular groove in aplane substantially orthogonal to the longitudinal axis of the stem 15.The shoulder 26 is at the transition between a region 33 of greater stemdiameter and a region 35 of narrower stem diameter. The groove 28 is inthe region 35 of narrower stem diameter and is of narrower diameter thanthe portions of stem 15 axially immediately either side of the groove28.

In other examples, the stem locking feature 25 may take different forms.The shoulder and/or the groove may, in some examples, not be continuous,i.e. may extend only part way round the circumference of the stem 15. Insuch cases, there may be several separate shoulders and/or groovesspaced around the circumference of the stem 15. The separate shouldersand/or grooves may give the stem locking feature 25 a castellatedappearance.

The stem 15 includes a stem enlarged portion 27. During installation ofthe fastener 11 in a workpiece 19 (see FIGS. 2 to 7 and FIGS. 8 to 13),a pulling force is applied to the stem 15 by an installation tool 51 atthe opposite end of the stem 15 from the enlarged portion 27. Thepulling force draws the stem 15 through the body 13, towards the headend of the body 13 (towards the left in the examples of FIGS. 1 to 7 andFIGS. 8 to 13). The stem enlarged portion 27 causes the shank 17 of thebody 13 to deform radially outwardly as the enlarged portion 27 passesthrough the shank 17, as illustrated in FIGS. 2 to 7 and FIGS. 8 to 13.This leads to the formation of a radially enlarged bulb on the blindside of the workpiece 19 (i.e. the opposite side of the workpiece 19from the side on which the installation tool 51 is applied). In theexamples illustrated in the Figures, the right-hand side of theworkpiece 19 is the blind side on which the enlarged bulb is formed.

FIG. 2 shows the fastener 11 of FIG. 1 after it has been inserted into aworkpiece 19 comprising two workpiece members 191 and 192. A tool 51 ispushed against the body head 21 and will be used to set the fastener 11in the workpiece 19 by applying a pulling force to the stem 15. Theskilled person will appreciate that the invention can be applied to twoas well as more than two workpiece members simultaneously depending onspecific applications.

FIGS. 3 to 7 illustrate the movements of the body 13, the stem 15 andthe workpiece 19 during the setting operation. In FIG. 3 the stemenlarged portion 27 has been drawn into the shank 17 of the body 13,causing the radially outward movement of the shank 17 described above.The shoulder 26 of the stem locking feature 25 is approaching theshoulder 23 that forms the body locking feature 23 in the illustratedexample.

When the stem 15 has been drawn sufficiently far through the body 13,the stem locking feature 25 and the body locking feature 23 will comeinto contact with one another. In the illustrated example, the shoulder26 of the stem 15 will come into contact with the shoulder 23 of thebody 13. This stage of the setting operation is illustrated in FIG. 4.

As the stem 15 is drawn still further through the body 13, a force isapplied to the body locking feature 23 via the stem locking feature 25.The force applied to the body locking feature 23 promotesdisplacement/deformation of at least part of the body 13. The verbs“displace” and “deform” (and their derivatives) in this context mean any“change in shape or form or configuration” and are intended to include(but are not limited to) the body 13 changing in shape or configurationby rotation or pivoting of one or more parts of the body 13 relative toother parts of the body; and one or more parts of the body 13 elongatingor contracting, e.g. by flow of material from one part of the body 13 toanother part of the body 13. As will be described in more detail below,the displacement/deformation of the body 13 gives rise to a lockingconnection between the body 13 and the stem 15.

In the illustrated example, the shoulder 26 of the stem 15 applies aforce to the shoulder 23 of the body 13 as the stem 15 is pulled by theinstallation tool. The force causes the body head 21 to deform/displace.

FIG. 5 illustrates the fastener 11 at a later stage in the settingprocess, after the body locking feature 23 and the stem locking feature25 have come into contact with each other and the body head 21 has begunto deform/displace. The force applied to the body 13 by the stem 15 hascaused the body head 21 to pivot about the point at which the radiallyenlarged head 21 and the body shank 17 are connected. The head 21 haspivoted such that the radially outermost part 37 of the head 21 is stillin contact with the workpiece 19, but an underhead recess 39 has beenformed, leaving a gap between the workpiece 19 and part of the underheadcontact surface 22 of the body head 21 that is radially inward of theradially outermost part 37. The underhead contact surface 22 no longerlies substantially in one plane, i.e. is no longer approximately flat.

Part of the shank 17 has been drawn through the aperture in theworkpiece 19 and out onto the tool-side of the workpiece 19. This may beas a result of the whole of the shank 17 moving further towards the toolside of the workpiece 19, or as a result of part of the shank 17deforming by elongating, depending on the extent of deformation of thetail end of the shank 17 through axial contraction and/or radialexpansion.

When the fastener 11 has reached the stage of installation illustratedin FIG. 5, the body locking feature 23 (shoulder 23 in the illustratedexample) is the radially innermost part 41 of the head 21. The shoulder23 has pivoted about the point at which the head 21 is connected to theshank 17. The shoulder 23 is still in contact with the stem lockingfeature 25 (shoulder 26 in the illustrated example).

When the head 21 has pivoted sufficiently relative to the body shank 17,the head 21 will be prevented by the installation tool 51 from pivotingfurther, as will be described in more detail below. Continued orincreased application of force to the stem 15 by the installation tool51 may thereafter cause other types of deformation of the head 21 (i.e.distinct from pivoting of the head 21). It may, in particular, causefurther displacement/deformation of the body locking feature 23(shoulder 23) into the groove 28 on the stem 15 (if possible), andcorresponding deformation of the head and/or shank material next to thebody locking feature 23 to enable the deformation of the body lockingfeature 23. This deformation arises as a consequence of a contact forceapplied by the tool 51 to the head 21, wherein the contact force has acomponent which acts radially inwardly. Such deformation of the shoulder23 to occupy more of the groove 28 is illustrated in FIG. 6.

There will come a point at which further force applied to the stem 15will cause the stem 15 to fracture at a break groove 43. After the stem15 has fractured, part of the stem 15 is left within the body 13, andpart of the stem 15 is removed by the tool. The fracturing of the stem15 at the break groove 43 is illustrated in FIG. 7. The fracturing ofthe stem 15 completes the fastener installation process.

The deformation of the body 13 (including the pivoting of the head 21)during setting of the fastener 11 gives rise to locking engagement ofthe body locking feature 23 and the stem locking feature 25. Inparticular, the deformation of the head 21 causes the shoulder 23 toenter the groove 28 in the outer surface of the stem 15.

The engagement of the body locking feature 23 and the stem lockingfeature 25 constrains the relative movement of the body 13 and the stem15. In particular, the fact that the shoulder 23 has entered the groove28 limits the extent to which the stem 15 can slide axially within thebore of the body 13. Thus, the engagement of the body locking feature 23and the stem locking feature 25 helps prevent the stem 15 from recoilingout of the body 13 when the stem 15 fractures at the break groove 43,and/or from being shaken loose from the body 13 by vibration of theworkpiece 19 after the installation process.

Retaining the stem 15 within the body 13 in this way may improve theshear strength of the fastener 11, e.g. the ability of the fastener toconstrain sliding of one workpiece member relative to the other in adirection substantially orthogonal to the longitudinal axis of thefastener 11, because of the strength of the material forming the stem15.

Retaining the stem 15 within the body 13 may alternatively oradditionally improve the tensile strength of the fastener, e.g. theability of the fastener 11 to prevent separation of the workpiecemembers in a direction substantially parallel to the longitudinal axisof the fastener 11.

As illustrated in FIGS. 2 to 7 and in FIGS. 8 to 13, the tool 51 forinstalling the fastener 11 has a contact face 53 (see, for example, FIG.2 or FIG. 8). In the illustrated examples, the contact face 53 is aconcave contact face. The approximate radius of curvature of the contactface 53 is smaller than the approximate radius of curvature of the outersurface of the body head 21. When the contact face 53 of the tool 51 isfirst pressed against the body head 21, the contact face 53 contacts thebody head 21 only on a first contact ring 55 near the radially outermostportion of the contact face 53. There is a gap between the radiallyinward portion of the contact face 53 and the radially inward portion ofthe body head 21, as illustrated in FIGS. 2 to 4 and FIGS. 8 to 11.

As the tool 51 pulls on the stem 15 to set the fastener 11 in position,the body head 21 pivots, as previously described. As the body head 21pivots, the approximate radius of curvature of the domed body head 21 asa whole decreases, and the radially inward portion of the body head 21approaches the radially inward portion of the contact face 53, closingthe gap between the radially inward portion of the contact face 53 andthe radially inward portion of the body head 21. The body head 21eventually pivots to such an extent that the head 21 cannot pivot anyfurther in that direction. This is because the radially inward portionof the head 21 comes into contact with the radially inward portion ofthe contact face 53 on a second contact ring 57, as illustrated in FIG.5 and FIG. 12.

When the body head 21 is in contact with the contact face 53 on thesecond contact ring 57 and the stem 15 is pulled further, the body head21 cannot pivot as it did previously. The reaction force that the secondcontact ring 57 exerts against the surface of the body head 21 overcomesthe force that tends to pivot the head 21. The reaction force has acomponent which is radially inward. The radially inward force applied tothe body head 21 helps promote the deformation (e.g. extension and/orrounding) of the shoulder 23 into the groove 28. In this way, an axialforce applied to the stem 15 by the tool 51 gives rise to a radiallyinward force applied to the body head 21. The radially inward forcepromotes deformation of the body locking feature 23 into the stemlocking feature 25, thereby helping to constrain relative moment of thebody 13 and the stem 15.

The tool 51 may be arranged such that the second contact ring 57 is ator near the most radially inward portion 54 of the contact face 53.Advantageously this may reduce marking of the body head 21 caused byapplication of the tool 51 to the fastener 11. The contact face 53 mayinclude a substantially flat portion at the most radially inward portion54, to further minimise marking of the body head 21 caused by thecontact face 53. The contact face 53 may be arranged such that its innerradius is approximately equal to the radius of the bore through the body13 for a given fastener 11.

As well as improving the strength of the fastening, the engagement ofthe body locking feature 23 and the stem locking feature 25 through thedeformation of the body head 21 helps make the installed fastener moretamper-proof.

As discussed above, during setting of the fastener 11 in a workpiece,the domed body head 21 pivots about the point at which it is connectedto the shank 17. As the head 21 pivots, the approximate radius ofcurvature of the head 21 as a whole decreases, and the radially inwardportion of the outer surface of the body head 21 approaches the radiallyinward portion of the contact face 53, closing the gap between theradially inward portion of the contact face 53 and the radially inwardportion of the body head 21. This deformation/displacement of the bodyhead 21 allows the radially outermost part 37 of the head 21 to remainin contact with the workpiece, maintaining a better seal of the bodyhead 21 against the workpiece. This effect is particularly advantageouswhen the fastener 11 is used with curved workpieces, as the radiallyoutermost part 37 of the head 21 pivots and stays in contact with thecurved surface of the workpiece.

In FIGS. 8 to 13 a fastener 11 according to an embodiment of theinvention is shown being set in a flat workpiece 119 rather than acurved workpiece 19 as illustrated in FIGS. 2 to 7. Indeed, the fastener11 may be used with various shapes of workpiece and is not intended tobe limited to use with a curved workpiece or a flat workpiece.

The steps of the setting process illustrated in FIGS. 8 to 13substantially correspond to the steps illustrated in FIGS. 2 to 7. InFIGS. 8 to 13, jaws 59 forming part of the installation tool 51 areshown. The jaws 59 (which were omitted from FIGS. 2 to 7) are includedfor illustrative purposes in FIGS. 8 to 13. The jaws 59 are used to gripa pulling portion of the stem 15 and to apply the pulling forcediscussed above.

As noted above, although in the illustrated examples the body head 21has a domed outer surface, in other examples, the body head may have adifferent profile, such as a flat top. Similarly, although theillustrated contact face 53 of the installation tool 51 is a concavecontact face, in other examples, the contact face may take a differentform, provided that there is a gap between the radially inward portionof the body head 21 and the radially inward portion of the contactsurface to allow the displacement/deformation necessary for the lockingto take place. The surface could for example take the form of a conicalor frustoconical surface, or a stepped surface.

In some embodiments, the shank 17 of the body 13 may include a voidedsection which may promote filling of the apertures in the workpiecemembers on installation of the fastener 11.

The materials of the body 13 and the stem 15 may be selected so thatdeformation of the body head 21 is promoted and deformation of the stem15 is resisted during the setting operation. The body 13 may for exampleinclude a relatively soft steel or other suitable material, and the stem15 may include a relatively hard steel or other suitable material.

1. A rivet for fastening workpiece members, the rivet comprising: a bodycomprising a shank extending in a first, longitudinal direction and ahead extending in a second, substantially transversal direction, a borebeing defined through the shank and the head; and, a stem comprising amain longitudinal portion arranged for insertion into said bore, and anenlarged portion arranged for radially expanding the shank when theenlarged portion is forced into said bore to fasten the workpiecemembers; the rivet being configured for insertion into respectiveapertures formed in the workpiece members; and the rivet being furtherconfigured such that when the workpiece members are fastened, the headof the body of the rivet undergoes displacement relative to the shank toengage with the stem to lock the stem in place relative to the bodyand/or the workpiece members.
 2. A rivet according to claim 1, whereinthe main longitudinal portion of the stem comprises at least one stemlocking feature to facilitate the engaging of the head of the body ofthe rivet with the stem.
 3. A rivet according to claim 2, wherein the atleast one stem locking feature comprises a stem shoulder and/or a stemgroove.
 4. A rivet according to claim 1, wherein the head of the body ofthe rivet comprises at least one body locking feature to facilitate theengaging of the head of the body of the rivet with the stem.
 5. A rivetaccording to claim 4, wherein the at least one body locking featurecomprises an inner body shoulder and/or an inner body groove.
 6. A rivetaccording to claim 1, wherein said head has an outer curved or domedprofile.
 7. A rivet according to claim 6, wherein said head has a flator curved underhead contact surface for abutment against a correspondingrespectively flat or curved surface of one of the workpiece members. 8.A rivet according to claim 1, wherein said displacement comprisespivoting of at least a portion of the head around a region of the headconstrained between a surface of one of the workpiece members and anouter rim of a tool for setting the rivet.
 9. A rivet according to claim1, wherein at least a substantial portion of the head undergoes yieldingto enable said displacement to lock the stem in place relative to thebody and/or the workpiece members.
 10. A rivet according to claim 1,wherein the body is formed as a single piece and/or is made of steel.11. A rivet according to claim 10, wherein the steel is in an annealedcondition.
 12. A rivet according to claim 1, wherein the stem is formedas a single piece and/or is made from steel.
 13. A rivet according toclaim 12, wherein the steel is in a hardened condition.
 14. A rivetaccording to claim 1, wherein the body shank has a plain cylindricalouter profile, or is splined.
 15. A rivet according to claim 14, whereinthe body shank includes a voided section.
 16. A rivet according to claim1, wherein the enlarged portion of the stem comprises a tapered sectionarranged to facilitate the radial expansion of at least part of the bodyshank.
 17. A workpiece comprising a rivet according to claim 1, whereinsaid workpiece further includes a curved surface for abutment with thehead of the body of the rivet.
 18. A method of fastening workpiecemembers using a rivet comprising a body having a shank extending in afirst, longitudinal direction and a head extending in a second,substantially transversal direction, a bore being defined through theshank and the head of the body, and a stem having a main longitudinalportion arranged for insertion into said bore, and an enlarged portionarranged for radially expanding the shank, the method comprising:inserting the rivet into respective apertures formed in the workpiecemembers; forcing the enlarged portion into said bore to fasten theworkpiece members; and, displacing the head of the body of the rivetrelative to the shank to engage with the stem to lock the stem in placerelative to the body and/or the workpiece members.
 19. A methodaccording to claim 18, wherein the step of displacing the head of thebody relative to the shank to engage with the stem to lock the stem inplace relative to the body and/or the workpiece member is performedimmediately subsequently to the step of forcing the enlarged portioninto the bore to fasten the workpiece members.
 20. A method according toclaim 19, wherein the step of displacing the head of the body relativeto the shank to engage with the stem to lock the stem in place relativeto the body and/or the workpiece member is performed immediatelysubsequently to, and as a result of a continuation of a same actionresponsible for, the step of forcing the enlarged portion into the boreto fasten the workpiece members.